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Warming-induced global water cycle changes pose a significant challenge to global ecosystems and human society. However, quantifying historical water cycle change is difficult owing to a dearth of direct observations, particularly over the ocean, where 77% and 85% of global precipitation and evaporation occur, respectively 1 – 3 . Air–sea fluxes of freshwater imprint on ocean salinity such that mean salinity is lowest in the warmest and coldest parts of the ocean, and is highest at intermediate temperatures 4 . Here we track salinity trends in the warm, salty fraction of the ocean, and quantify the observed net poleward transport of freshwater in the Earth system from 1970 to 2014. Over this period, poleward freshwater transport from warm to cold ocean regions has occurred at a rate of 34–62 milli-sverdrups (mSv = 10 3  m 3  s −1 ), a rate that is not replicated in the current generation of climate models (the Climate Model Intercomparison Project Phase 6 (CMIP6)). In CMIP6 models, surface freshwater flux intensification in warm ocean regions leads to an approximately equivalent change in ocean freshwater content, with little impact from ocean mixing and circulation. Should this partition of processes hold for the real world, the implication is that the historical surface flux amplification is weaker (0.3–4.6%) in CMIP6 compared with observations (3.0–7.4%). These results establish a historical constraint on poleward freshwater transport that will assist in addressing biases in climate models. A study uses a temperature-percentile water mass framework to analyse warm-to-cold poleward transport of freshwater in the Earth system, and establishes a constraint to help address biases in climate models.

For decades oceanographers have understood the Atlantic meridional overturning circulation (AMOC) to be primarily driven by changes in the production of deep-water formation in the subpolar and subarctic North Atlantic. Indeed, current Intergovernmental Panel on Climate Change (IPCC) projections of an AMOC slowdown in the twenty-first century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep-water formation. The motivation for understanding this linkage is compelling, since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic Program (OSNAP), to provide a continuous record of the transbasin fluxes of heat, mass, and freshwater, and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA) at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014, and the first OSNAP data products are expected in the fall of 2017.

Changes in the global water cycle critically impact environmental, agricultural, and energy systems relied upon by humanity (Jiménez Cisneros et al 2014 Climate Change 2014: Impacts, Adaptation, and Vulnerability (Cambridge: Cambridge University Press)). Understanding recent water cycle change is essential in constraining future projections. Warming-induced water cycle change is expected to amplify the pattern of sea surface salinity (Durack et al 2012 Science 336 455-8). A puzzle has, however, emerged. The surface salinity pattern has amplified by 5%-8% since the 1950s (Durack et al 2012 Science 336 455-8, Skliris et al 2014 Clim. Dyn. 43 709-36) while the water cycle is thought to have amplified at close to half that rate (Durack et al 2012 Science 336 455-8, Skliris et al 2016 Sci. Rep. 6 752). This discrepancy is also replicated in climate projections of the 21st century (Durack et al 2012 Science 336 455-8). Using targeted numerical ocean model experiments we find that, while surface water fluxes due to water cycle change and ice mass loss amplify the surface salinity pattern, ocean warming exerts a substantial influence. Warming increases near-surface stratification, inhibiting the decay of existing salinity contrasts and further amplifying surface salinity patterns. Observed ocean warming can explain approximately half of observed surface salinity pattern changes from 1957-2016 with ice mass loss playing a minor role. Water cycle change of 3.6% ± 2.1% per degree Celsius of surface air temperature change is sufficient to explain the remaining observed salinity pattern change.

Greenhouse gases and aerosols play a major role in controlling global climate change. Greenhouse gases drive a radiative imbalance which warms the ocean, while aerosols cool the ocean. Since 1980, the effective radiation felt by the planet due to anthropogenic aerosols has leveled off, global ocean cooling due to aerosols has decelerated, and greenhouse gas‐driven ocean warming has accelerated. We explore the deceleration of aerosol‐driven ocean cooling by quantifying a time‐ and spatially varying ocean heat uptake efficiency, defined as the change in the rate of global ocean heat storage per degree of cooling surface temperature. In aerosol‐only simulations, ocean heat uptake efficiency has decreased by 43 ± 14% since 1980. The tropics and sub‐tropics have driven this decrease, while the coldest fraction of the ocean continues to sustain cooling and high ocean heat uptake efficiency. Our results identify a growing trend toward less efficient ocean cooling due to aerosols. Plain Language Summary The composition of the atmosphere has a major impact on our climate. Greenhouse gases warm the planet, while aerosols (i.e., suspensions of particles in the atmosphere) cool the planet, and most of this change is absorbed by the oceans. Since 1980, the rate of cooling of the planet due to aerosols has plateaued. In the past few decades, the ocean has begun to equilibrate to this change, and this work explores where and when this equilibration has occurred in the ocean based on global climate models. To understand this change, we use an “ocean heat uptake efficiency” metric which describes how much additional heat builds up in the ocean for a given degree of surface temperature gain (or loss). We find that the ocean is cooling more slowly given a degree of surface cooling due to aerosols compared to the pre‐1980s. This change is largely driven by the tropics and sub‐tropics, where the ocean has stopped cooling in response to aerosol‐driven negative surface temperatures. Polar and sub‐polar regions, however, continue to cool due to aerosols. These changes are occurring alongside accelerating greenhouse gas‐driven warming, suggesting that the relative role of aerosols in cooling our climate is weakening. Key Points Since 1980, aerosol‐driven ocean cooling has decelerated substantially, alongside a drop in ocean heat uptake efficiency The drop in ocean heat uptake efficiency is limited to the tropics, which may have equilibrated to ongoing aerosol‐driven radiative forcing Air‐sea fluxes into the coldest fraction of the ocean continue to offset greenhouse gas‐driven ocean warming

Mixing along sloping isopycnals plays a key role in the transport and uptake of heat and carbon by the ocean. This mixing is quantified by a lateral diffusivity, which can be measured by tracking the lateral spreading of point release tracer patches. We present a definition for the area of a tracer patch, the time derivative of which provides the lateral diffusivity. To accurately estimate the diffusivity, an ensemble mean concentration field of many tracer release experiments is required. We use numerical experiments to quantify how accurately the “true” lateral diffusivity (obtained from the ensemble mean concentration field) can be estimated from a single tracer release experiment (one ensemble member). To simulate observational campaigns, we also estimate the diffusivity from a single tracer release that is spatially and/or temporally subsampled, quantifying how the error between the estimated diffusivity and the true diffusivity grows as this sampling resolution worsens. We perform these numerical experiments in a two-layer quasigeostrophic model of turbulent flow on a β plane, using an ensemble of 50 passive tracer release experiments, each initialized as a 2D Gaussian but with differing realizations of the turbulent flow. We find that the diffusivity estimates from the single tracer releases have a relative root-mean-square error (RMSE) of 1.43% from the true diffusivity. Subsampling a single tracer release experiment every 956 km increases the relative RMSE from the true diffusivity to 3.1%; also subsampling every 277 days raises this figure to 6.5%.

The Southern Hemisphere westerly winds have intensified in recent decades associated with a positive trend in the southern annular mode (SAM). However, the response of the Antarctic Circumpolar Current (ACC) transport and eddy field to wind forcing remains a topic of debate. This study uses global eddy-permitting ocean circulation models driven with both idealized and realistic wind forcing to explore the response to interannual wind strengthening. The response of the barotropic and baroclinic transports and eddy field of the ACC is found to depend on the spatial pattern of the changes in wind forcing. In isolation, an enhancement of the westerlies over the ACC belt leads to an increase of both barotropic and baroclinic transport within the ACC envelope, with lagged enhancement of the eddy kinetic energy (EKE). In contrast, an increase in wind forcing near Antarctica drives a largely barotropic change in transport along closed f / H contours (“free mode”), with little change in eddy activity. Under realistic forcing, the interplay of the SAM and the El Niño–Southern Oscillation (ENSO) influences the spatial distribution of the wind anomalies, in particular the partition between changes in the wind stress over the ACC and along f / H contours. This study finds that the occurrence of a negative or positive ENSO during a positive SAM can cancel or double the wind anomalies near Antarctica, altering the response of the ACC and its eddy field. While a negative ENSO and positive SAM favors an increase in EKE, a positive ENSO and positive SAM lead to barotropic transport changes and no eddy response.

This study provides observation-based estimates, determined by inverse methods, of horizontal and isopycnal eddy diffusion coefficients K H and K I , respectively, the small-scale mixing coefficient D , and the diathermohaline streamfunction Ψ. The inverse solution of Ψ represents the ocean circulation in Absolute Salinity S A and Conservative Temperature Θ coordinates. The authors suggest that the observation-based estimate of Ψ will be useful for comparison with equivalent diagnostics from numerical climate models. The estimates of K H and K I represent horizontal eddy mixing in the mixed layer and isopycnal eddy mixing in the ocean interior, respectively. This study finds that the solution for D and K H are comparable to existing estimates. The solution for K I is one of the first observation-based global and full-depth constrained estimates of isopycnal mixing and indicates that K I is an order of magnitude smaller than K H . This suggests that there is a large vertical variation in the eddy mixing coefficient, which is generally not included in ocean models. With ocean models being very sensitive to the choice of isopycnal mixing, this result suggests that further investigation of the spatial structure of isopycnal eddy mixing from observations is required.

Water Mass Transformation (WMT) theory provides conceptual tools that in principle enable innovative analyses of numerical ocean models; in practice, however, these methods can be challenging to implement and interpret, and therefore remain under‐utilized. Our aim is to demonstrate the feasibility of diagnosing all terms in the water mass budget and to exemplify their usefulness for scientific inquiry and model development by quantitatively relating water mass changes, overturning circulations, boundary fluxes, and interior mixing. We begin with a pedagogical derivation of key results of classical WMT theory. We then describe best practices for diagnosing each of the water mass budget terms from the output of Finite‐Volume Generalized Vertical Coordinate (FV‐GVC) ocean models, including the identification of a non‐negligible remainder term as the spurious numerical mixing due to advection scheme discretization errors. We illustrate key aspects of the methodology through the analysis of a polygonal region of the Greater Baltic Sea in a regional demonstration simulation using the Modular Ocean Model v6 (MOM6). We verify the convergence of our WMT diagnostics by brute‐force, comparing time‐averaged (“offline”) diagnostics on various vertical grids to timestep‐averaged (“online”) diagnostics on the native model grid. Finally, we briefly describe a stack of xarray‐enabled Python packages for evaluating WMT budgets in FV‐GVC models (culminating in the new xwmb package), which is intended to be model‐agnostic and available for community use and development. Plain Language Summary A useful tool for characterizing ocean variability and change is water mass analysis, in which the ocean is decomposed into parcels with distinct properties (such as their temperature, density, or dissolved oxygen concentrations). Water Mass Transformation (WMT) theory provides a concise equation for the evolution of these water masses, which can be used to identify the various processes that act to increase or decrease the total mass (or size) of each parcel. In practice, however, calculating the terms in these WMT budget equations from ocean model simulation output is technically challenging, limiting the creativity of applications in the literature. We review the fundamentals of WMT theory, explain how to calculate WMT budgets based on the output of a generic ocean model simulation (with examples generated from a widely used code), and summarize a new publicly available software for doing such calculations: the xwmb Python package. Key Points Water mass budgets provide insights into the processes transforming material water mass properties and how they relate to circulation Spurious water mass transformations due to advection scheme errors can be quantified by combining mass and tracer budget diagnostics We describe best practices for model analysis and present a novel Python stack for model‐agnostic and out‐of‐memory regional calculations

The global water cycle leaves an imprint on ocean salinity through evaporation and precipitation. It has been proposed that observed changes in salinity can be used to infer changes in the water cycle. Here salinity is characterized by the distribution of water masses in salinity coordinates. Only mixing and sources and sinks of freshwater and salt can modify this distribution. Mixing acts to collapse the distribution, making saline waters fresher and fresh waters more saline. Hence, in steady state, there must be net precipitation over fresh waters and net evaporation over saline waters. A simple model is developed to describe the relationship between the breadth of the distribution, the water cycle, and mixing—the latter being characterized by ane-folding time scale. In both observations and a state-of-the-art ocean model, the water cycle maintains a salinity distribution in steady state with a mixing time scale of the order of 50 yr. The same simple model predicts the response of the salinity distribution to a change in the water cycle. This study suggests that observations of changes in ocean salinity could be used to infer changes in the hydrological cycle.

Tasman leakage, the westward flow of thermocline waters south of Australia from the Pacific to the Indian Ocean, is one of the lesser‐studied of the inter‐ocean exchanges. Here, some of the properties of the Tasman leakage are inferred from Lagrangian particles integrated using the three‐dimensional velocity fields of the 1/10 degree resolution OFES model. The mean Tasman leakage in this model is 4.2 Sv, with a standard deviation of 4.3 Sv. The heat flux associated with this leakage lies in the range 0.08–0.18 PW. There is large variability in the Tasman leakage on both sub‐weekly and inter‐annual scales, but no trend over the 1983–1997 period. Despite the large weekly variability, with peaks of more than 20 Sv, it appears that less than half of the Tasman leakage is carried within eddies. Key Points In the OFES model, Tasman leakage is on average 4.2 Sv, 30% of ITF flux Tasman leakage heat flux is between 0.08 and 0.18 PW, which is not insignificant Less then half of Tasman leakage is within eddies, which affects monitoring